(R)-selective nitroaldol reaction catalysed by proteins of the cupin superfamily

ABSTRACT

The present invention relates to a method for producing chiral β-nitro alcohol compounds. The invention relates in particular to an (R)-selective cupin-nitroaldolase, which enantioselectively can catalyze the Henry reaction, wherein an aldehyde or ketone compound is converted to the corresponding β-nitro alcohol compound in the presence of a nitroalkane compound and a cupin-nitroaldolase.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No. 14/317,169, filed Dec. 8, 2016, which is a national stage entry under 35 U.S.C. § 371 from PCT International Application No. PCT/EP2015/063195 filed Jun. 12, 2015, which claims the benefit to and priority of European Patent Application No. 14172208.2, filed Jun. 12, 2014, the subject matter of all of which is incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to a method for producing chiral β-nitro alcohol compounds, wherein an aldehyde or ketone compound is converted to the corresponding β-nitro alcohol compound in the presence of a nitroalkane compound and a cupin-nitroaldolase. The invention relates in particular to a (R)-selective cupin-nitroaldolase which enantioselectively catalyzes the Henry reaction.

BACKGROUND ART

Biocatalytic processes have become very important to the chemical industry. Of particular importance is the use of enzymes, when the properties of biocatalysts enable either of the two enantiomers in chemical reaction with chiral or prochiral compounds to be reacted or formed preferentially.

Essential requirements for utilizing these favorable properties of enzymes are their low-cost availability in sufficient amounts, as required in industrial processes, a sufficiently high reactivity, selectivity and high stability under the realistic conditions of the industrial process.

β-nitro alcohols are precursors for β-amino alcohols, which are important chiral building blocks for the synthesis of bioactive compounds, such as ephedrine, bestatin and sphingosine, used as pharmaceutical ingredients. The nitroaldol or Henry reaction is one of the classical named reactions in organic synthesis for C—C bond formation. Due to the potential to create up to two new chiral centers it is of fundamental importance for synthetic applications to be able to perform the nitroaldol addition enantio- and stereoselectively. Although the reaction has been known for more than a century (Henry, 1895), stereospecific protocols utilizing non-enzymatic organocatalysts or chiral metal catalysts have been developed only recently. The development of these methods is impressive, but they still share a number of disadvantages, including long reaction times and sometimes extreme reaction conditions in the case of metal catalysts, or insufficient selectivities in the case of organocatalysts.

In the past decade, the first asymmetric biocatalytic nitroaldol reaction was discovered for the hydroxynitrile lyase from the tropical rubber tree Hevea brasiliensis (HbHNL) (Purkarthofer et al., Angew Chem Int Ed Engl. 2006 45(21):3454-6, Gruber-Khadjawi et al., Adv. Synth. Catal. 2007, 349, 1445-1450, Yuryev, R.; et al., Biocatal. Biotransform., (2010) 28, 348; Yuryev, R.; et al.; Chemcatchem, (2010) 2, 981)).

The (S)-selective MeHNL from Manihot esculenta, which like HbHNL belongs to the α/β-hydrolase superfamily, is also capable of catalyzing the (S)-selective nitroaldol reaction, albeit with lower activity and selectivity.

The first (R)-selective HNL, which catalyzes the (R)-selective HNL-catalyzed Henry reaction is AtHNL from Arabidobsis thaliana (Fuhshuku et al. J. Biotechnol. 2011, 153, 153-159), which belongs also to the α/β-hydrolase superfamily like the (S)-selective nitroaldolases.

In contrast, activity in the nitroaldol reaction has not been shown so far for the (R)-selective hydroxynitrile lyase from Prunus amygdalus (PaHNL), which belongs to a different protein fold.

Unfortunately, however, the enantiomeric excess of the reaction product of AtHNL decreases significantly without increase of yield during prolonged reaction times (from 2 h to 4 h) at the reported reaction conditions (Fuhshuku et al. 2011).

Asano and coworkers achieved the highest enantioselectivity for benzaldehyde and MeNO₂ in a biphasic system at pH 7 with 50% n-butyl acetate (30% yield and 91% ee) applying 40 mg AtHNL per mmol benzaldehyde. Yield and enantiomeric excess were further slightly improved by applying larger amounts of enzyme (100 mg per mmol of substrate). Depending on the substrate and reaction system yields up to 60% or enantiomeric excess up to 96% could be obtained applying 40 mg of AtHNL per mmol of substrate and a reaction time of 2 h. However, not coexistent under the same reaction conditions.

However, the enantiomeric excess of the reaction product of AtHNL decreases significantly without increase of yield during prolonged reaction times (from 2 h to 4 h) at the reported reaction conditions. Nitroethane was not used.

Gotor and coworkers reported the protein-mediated catalysis of the nitroaldol reaction by the carrier protein bovine serum albumin (BSA) in water, which can be categorized as organocatalysis because the observations of the scientists led to the conclusion of unspecific protein catalysis. (Busto, E.; Gotor-Fernandez, V.; Gotor, V., Org. Process Res. Dev., (2011) 15, 236). Biocatalytic nitroaldol reactions were also reported with enzymes (for a review see Milner, S. E.; et al., Eur. J. Org. Chem, (2012), 3059), such as a transglutaminase, (Tang, R. C.; et al., J. Mol. Catal. B: Enzym., (2010) 63, 62) a hydrolase, (Wang, J. L.; et al., J. Biotechnol., (2010) 145, 240) a protease, (Lopez-Iglesias, M.; et al., Adv. Synth. Catal., (2011) 353, 2345) lipases, (Le, Z.-G.; et al., Green Chem. Lett. Rev., (2013) 6, 277; Xia, W.-J.; et al., Molecules, (2013) 18, 13910), an acylase (Xia, W.-J.; et al., Molecules, (2013) 18, 13910) and a glucoamylase (Gao, N.; et al., RSC Adv., (2013) 3, 16850), but in all cases the reactions were not enantioselective or no data about enantioselectivity were provided.

Thus, so far only plant HNLs with α/β-hydrolase fold were capable of catalysing the enantioselective nitroaldol reaction.

Another approach is a chemo-enzymatic approach, in which the chemically synthesized mixture of stereoisomers is separated by enzymatic kinetic resolution, e.g. using hydrolases. However, the major drawback of kinetic resolution in general is the limitation of the yield to a maximum of 50%.

Thus, there is still the need for new nitroaldolases, which can enantioselectively catalyze the Henry reaction.

Recently, the discovery of new bacterial HNLs with cupin fold has been reported (Hajnal, I.; et al., Febs J., (2013) 280, 5815; Hussain, Z.; et al., Appl. Environ. Biotechnol., (2012) 78, 2053), however displaying only very low specific activity. One of the new enzymes, GtHNL, was characterized in detail and its structure was solved (Hajnal, I.; et al., Febs J., (2013) 280, 5815; Lyskowski, A.; et al., Acta Crystallogr. F, (2012) 68, 451). It is a small metal-dependent mono-cupin with a molecular weight of ˜15 kDa, which forms a tetramer.

SUMMARY OF INVENTION

It is the objective of the present invention to provide an enhanced method for producing p-nitro alcohol compounds. The method comprises the steps of providing an aldehyde or ketone compound and converting the compound to the corresponding p-nitro alcohol compound in the presence of a nitroalkane compound and a cupin-nitroaldolase. The invention relates in particular to an (R)-selective cupin-nitroaldolase, which can enantioselectively catalyze the Henry reaction.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1A: Cartoon representation of one cupin monomer of the structure of GtHNL (PDB-code: 4BIF). FIG. 1B is a table showing a multi sequence alignment of SEQ ID NO: 1, NO:3 and NOS: 5-10.

FIG. 2: Nitroaldol reaction of AcHNL and GtHNL and the triple variants of GtHNL (GtHNL-A40H/V42T/Q110H, short: GtHNLmut) and AcHNL (AcHNL-A40H/V42T/Q110H, short: AcHNLmut) with benzaldehyde (20 tmol) and nitromethane (1 mmol) in a biphasic system with 50% TBME. Cleared lysate (the amount of enzyme/mmol BA refers to the enzyme itself assuming that ˜50% of total lysate protein are HNL) or purified enzyme was used. Conversion and ee values measured after 6 h reaction time.

FIG. 3: Nitroaldol reaction of the triple variant GtHNL-A40H/V42T/Q110H (short: GtHNLmut) with benzaldehyde (20 tmol) and nitromethane (1 mmol) in a biphasic system with 50% TBME, and different protein to substrate ratios. AtHNL was used as reference. Conversion and ee values were measured after 2 h and 24 h reaction time.

FIG. 4: Nitroaldol reaction of several cupin-nitroaldolases with benzaldehyde (20 mol) and nitroethane (1 mmol) in a biphasic system with 50% TBME. Conversion and ee values were measured after 2 h, 4 h and 24 h reaction time.

FIG. 5: Protein sequence of cupin-nitroaldolases

SEQ ID NO: 1GtHNL (Granulicella tundricola MP5ACTX9, Uniprot E8WYN5)

SEQ ID NO:2 Protein sequence of GtHNL triple variant (A40H, V42T, Q110H)

SEQ ID NO:3 Protein sequence of AcHNL (Acidobacterium capsulatum ATCC 51196; Uniprot C1F951)

SEQ ID NO:4 Protein sequence of AcHNL triple variant (A40H, V42T, Q110H)

SEQ ID NO:5 Protein sequence of a cupin 2 conserved barrel domain protein (Uniprot B8ENI4)

SEQ ID NO:6 Protein sequence of a cupin 2 conserved barrel domain protein (Uniprot A5G162)

SEQ ID NO:7 Protein sequence of a cupin 2 conserved barrel domain protein (Uniprot C6D499)

SEQ ID NO:8 Protein sequence of an uncharacterized protein (Uniprot C1D3E9)

SEQ ID NO:9 Protein sequence of a cupin 2 conserved barrel domain protein (Uniprot A6U7V5)

SEQ ID NO: 10 Protein sequence of a cupin 2 conserved barrel domain protein (Uniprot F8IF03),

DESCRIPTION OF EMBODIMENTS

In a first aspect, the invention relates to a method for producing a β-nitro alcohol compound, wherein an aldehyde or ketone compound is converted to the corresponding β-nitro alcohol compound in the presence of a nitroalkane compound and a cupin-nitroaldolase.

In another aspect, the invention relates to a method as described above, wherein the cupin-nitroaldolase comprises a conserved barrel domain according to FIG. 1A.

In another aspect, the invention relates to a method as described above, wherein the cupin-nitroaldolase comprises a conserved barrel domain of the cupin superfamily with a PFAM accession CL0029.

In another aspect, the invention relates to a method as described above, wherein the cupin-nitroaldolase comprises a conserved barrel domain of the cupin 2 family with a PFAM accession PF07883.

In another aspect, the invention relates to a method as described above, wherein the cupin-nitroaldolase belongs to the RmlC-like cupin superfamily with a SCOP accession 51182.

In another aspect, the invention relates to a method as described above, wherein the cupin-nitroaldolase comprises a RmlC-like jelly roll fold (IPR014710) according to the InterPro protein families database.

In another aspect, the invention relates to a method as described above, wherein the cupin-nitroaldolase comprises an RmlC-like cupin domain (IPRO11051) according to the InterPro protein families database.

In another aspect, the invention relates to a method as described above, wherein the cupin-nitroaldolase comprises a cupin 2, conserved barrel domain (IPR013096) according to the InterPro protein families database.

In another aspect, the invention relates to a method as described above, wherein a compound of formula I is reacted with a compound of formula II in the presence of a cupin-nitroaldolase to yield a 3-nitro alcohol compound of formula III

wherein R¹ and R² are independently from one another H, C₁₋₂₀alkyl, C₂₋₂₀alkenyl, or C₂₋₂₀alkynyl, C₃₋₁₀cycloalkyl, C₄₋₂₀cycloalkylalkyl, C₆₋₁₄aryl, C₇₋₂₀arylalkyl, 3-14 membered heterocycloalkyl, 4-20 membered heterocycloalkylalkyl, 5-20 membered heteroaryl or 6-20 membered heteroarylalkyl, optionally substituted by one or more R^(a); R³ and R⁴ are independently from one another H or C₁₋₂₀alkyl, optionally substituted by one or more R^(a); and each R^(a) is independently H, halogen, —CF₃, —OR^(b), —NR^(b)R^(b), —(CH₂)_(n)COOR^(b), —(CH₂)_(n)C(═O)R^(b), —(CH₂)_(n)CONR^(b)R^(b), C₁₋₂₀alkyl, C₂₋₂₀alkenyl, or C₂₋₂₀alkynyl; and each R^(b) is independently H or optionally substituted C₁₋₂₀alkyl, C₂₋₂₀alkenyl, or C₂₋₂₀alkynyl; and n is 0, 1, 2 or 3.

An alkyl group, if not stated otherwise, denotes a linear or branched C₁₋₂₀alkyl, preferably a linear or branched chain of one to twenty carbon atoms, optionally substituted. An alkyl group may be substituted by one or more R^(a).

An alkenyl group, if not stated otherwise, denotes a partially unsaturated linear or branched C₂₋₂₀alkenyl, preferably a linear or branched chain of two to twenty carbon atoms that contains at least one double bond, optionally substituted. An alkenyl group may be substituted by one or more R^(a).

An alkynyl group, if not stated otherwise, denotes a partially unsaturated linear or branched C₂₋₂₀alkynyl, preferably a linear or branched chain of two to twenty carbon atoms that contains at least one triple bond, optionally substituted. An alkynyl group may be substituted by one or more R^(a).

A cycloalkyl group denotes a monocyclic non-aromatic hydrocarbon ring containing three to ten carbon atoms, preferably four to six carbon atoms, or a bicyclic non-aromatic hydrocarbon ring system containing seven to ten carbon atoms, preferably seven carbon atoms, wherein the cycloalkyl group optionally comprises one or more double or triple bonds, optionally substituted. A cycloalkyl group may be substituted by one or more R^(a).

A heterocycloalkyl group denotes a monocyclic non-aromatic hydrocarbon ring containing three to fourteen carbon atoms, preferably four to eight carbon atoms, or a bicyclic non-aromatic hydrocarbon ring system containing seven to fourteen carbon atoms, preferably eight to ten carbon atoms, wherein in the heterocycloalkyl group one or more of the carbon atoms of the hydrocarbon ring or ring system is replaced by a group selected from the group comprising —N—, —O—, —S—, —S(O)—, —S(O)₂—, —Si— and —P—; wherein the heterocycloalkyl group optionally comprises one or more double bonds, optionally substituted. A heterocycloalkyl group may be substituted by one or more R^(a).

An aryl group preferably denotes a mono-, bi- tri- or tetracyclic, preferably monocyclic aromatic hydrocarbon group having six to fourteen carbon atoms; the aryl group is preferably phenyl, optionally substituted, optionally substituted. An aryl group may be substituted by one or more R^(a).

A heteroaryl group denotes an aromatic 5- or a 6-membered monocyclic hydrocarbon group wherein at least one of the carbon atoms is replaced by a heteroatom like O, N, and/or S, and wherein the aromatic monocyclic 5- or 6-membered cyclic hydrocarbon group is optionally fused to a further monocyclic 5- to 7-membered, preferably 5- or 6-membered, aromatic or nonaromatic hydrocarbon ring, wherein in the further monocyclic aromatic or nonaromatic hydrocarbon ring one or more, preferably one or two carbon atoms may be replaced by a heteroatom like O, N, and/or S, optionally substituted. A heteroaryl group may be substituted by one or more R^(a).

A halogen group is chlorine, bromine, fluorine or iodine.

Optionally substituted as used herein refers to a substituent selected from the group consisting of halogen, —OH, —OCH₃, —CN, carbonyl and carboxyl, C₁₋₂₀alkyl, C₂₋₂₀alkenyl and C₂₋₂₀alkynyl next to substituted and unsubstituted C₆₋₁₄aryl or C₅₋₁₄heteroaryl residues.

The method can be carried out in a mono- or biphasic system or in an emulsion.

The monophasic reaction solution comprises an aqueous or an organic solvent.

Appropriate aqueous solutions are for example water, a cupin-nitroaldolase containing solution, or a buffer solution. Examples for buffer solutions are phosphate buffer, citrate buffer, acetate buffer, borate buffer, MES, HEPES, Tris buffer, or mixtures thereof. The pH of these solutions can be between pH 2 and 9, preferably from 4 to 7.

Appropriate organic solutions can be slightly water-miscible or water immiscible aliphatic or aromatic hydrocarbons, which are optionally halogenated, alcohols, ethers or esters or mixture thereof or the substrate itself. Suitable are for example, but not limited to ethyl acetate, butyl acetate, methyl tert-butyl ether, diisopropyl ether, dibutyl ether, carbon tetrachloride, benzene, toluene, cyclohexane, hexadecane, hexane, heptane, chloroform, xylene, pentanol, hexanol, octanol and dodecanol, DMF, DMSO, acetonitrile, nitromethane, nitroethane, or mixtures thereof. Also applicable are neoteric solvents, which refers to ionic liquids and supercritical fluids.

The advantages of conducting bioconversions in aqueous-organic solvent two-liquid phase systems are well known in the art. The biphasic system consists of two phases mutually not miscible, e.g. an aqueous and an organic phase.

In a further aspect, the invention relates to a method as described above, wherein the reaction is carried out in a mono- or biphasic system or in an emulsion.

In a further aspect the invention relates to a method as described above, wherein the biphasic system comprises aqueous and organic solution as described herein.

The cupin-nitroaldolases of the invention may be present here either as purified enzyme or as a whole cell suspension or contained in a cell free lysate or in immobilized form, for example on a support such as Celite®, Avicel, etc. or as cross-linked enzyme aggregate (CLEA).

The conversion moreover takes place at temperatures of from −10° C. to +50° C., preferably at 0° C. to 35° C.

The choice of applicable electrophiles ranges from aromatic to heteroaromatic and aliphatic aldehydes. Depending on the substrate and reaction systems yields up to 97.3% or enantiomeric excess >99% could be obtained by the inventive method.

In a further aspect, the invention relates to a method as described above, wherein the β-nitro alcohol compound is obtained with at least 55%, preferably with at least 60%, more preferred with at least 75% enantiomeric excess (e.e.).

In a further aspect, the invention relates to a method as described above, wherein the β-nitro alcohol compound is obtained with a conversion rate of at least 10%, preferably with at least 20%, more preferred with at least 50%.

Proteins of the cupin superfamily of proteins (PFAM: CL0029) contain a conserved beta barrel domain composed of 10 to 12 anti-parallel beta-strands (FIG. 1A). Cupa is the latin term for barrel. In the structural classification of proteins database (SCOP) they are classified as the RmlC-like cupins superfamily [51182] within the double-stranded β-helix fold. The cupin fold is found in a wide variety of enzymes, but also non-enzymatic proteins. The cupin domain can be found once, twice or more in a protein structure, either alone or in combination with other domains. Although proteins in the cupin superfamily show very low overall sequence similarity, they all contain two short but partially conserved cupin sequence motifs separated by a less conserved intermotif region that varies both in length and amino acid sequence (FIG. 1). Proteins of the cupin superfamily have a wide range of biological functions in archaea, bacteria and eukaryotes.

Cupins are structurally conserved and usually contain two conserved motifs, G-(X)₅—H—X—H—(X)_(3,4)-E-(X)₆-G (motif 1) and G-(X)₅—P—X-G-(X)₂—H—(X)₃—N (motif 2), the overall sequence identity is low among members of this superfamily. The two motifs also include the residues for metal binding. Most cupins are metal-binding proteins that bind divalent cations such as iron, zinc, manganese, copper, nickel or cadmium. The metal is usually involved in the enzymatic reaction either directly in the reaction mechanism or at least via an interaction with the substrate.

In a further aspect, the invention relates to a cupin-nitroaldolase, which is capable to catalyze the asymmetric 3-nitro alcohol reaction with a conversion rate of at least 10%, preferably of at least 20%, more preferred of at least 50%.

In a further aspect, the invention relates to a cupin-nitroaldolase, wherein the cupin-nitroaldolase comprises a conserved barrel domain according to FIG. 1A.

In a further aspect, the invention relates to a cupin-nitroaldolase, wherein the cupin-nitroaldolase comprises a conserved barrel domain of the cupin superfamily with a PFAM accession CL0029.

In a further aspect, the invention relates to a cupin-nitroaldolase, wherein the cupin-nitroaldolase comprises a conserved barrel domain of the cupin 2 family with a PFAM accession PF07883.

In a further aspect, the invention relates to a cupin-nitroaldolase, wherein the cupin-nitroaldolase belongs to the RmlC-like cupin superfamily with a SCOP accession 51182.

In another aspect, the invention relates to a cupin-nitroaldolase, wherein said cupin-nitroaldolase comprises a RmlC-like jelly roll fold (IPR014710) according to the InterPro protein families database.

In another aspect, the invention relates to a cupin-nitroaldolase, wherein the cupin-nitroaldolase comprises an RmlC-like cupin domain (IPR011051) according to the InterPro protein families database.

In another aspect, the invention relates to a cupin-nitroaldolase, wherein the cupin-nitroaldolase comprises a cupin 2, conserved barrel domain (IPR013096) according to the InterPro protein families database.

A further aspect of the invention is a cupin-nitroaldolase as described above, which is a recombinant cupin-nitroaldolase.

A further aspect of the invention is a cupin-nitroaldolase as described above, comprising at least one, specifically at least two, specifically at least three, specifically at least four, specifically at least five or more amino acids modifications.

According to the invention, the term “modification” means a deletion or substitution or insertion of at least one amino acid. Specifically, the modification is a substitution of one amino acid.

In a specific embodiment of the invention, the cupin-nitroaldolase comprises one, two or three amino acid substitutions.

According to the embodiment of the invention, any amino acid can be selected to substitute the amino acid of the wild type sequence.

The substituted amino acids are selected from arginine, lysine, histidine and threonine. Specifically, said substituted amino acids are arginine, histidine or threonine.

In a further aspect, the invention relates to a cupin-nitroaldolase as described above, wherein the amino acid sequence of the cupin-nitroaldolase is at least 85%, at least 86%, at least 87%, at least 88%, at least 89%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, or at least 98% identical to the respective wild type enzyme.

In a further aspect, the invention relates to a cupin-nitroaldolase as described above, which is modified at any one of positions 40, 42 and/or 110 according to the numbering of SEQ ID NO: 1, SEQ ID NO:3 (FIG. 1B).

In a further aspect, the invention relates to a cupin-nitroaldolase as described above, which is modified at any one of positions 40, 42 and/or 110 according to the numbering of SEQ ID NO:5, SEQ ID NO:6, SEQ ID NO:7, SEQ ID NO:8, SEQ ID NO:9 or SEQ ID NO:10 (FIG. 1B).

According to the embodiment of the invention, the modification at any of positions 40, 42 and/or 110 are amino acid substitutions.

According to a further embodiment, the cupin-nitroaldolase variant of the invention contains one, two or three substitutions at any of positions 40, 42 and 110 of the conserved barrel domain.

In a further aspect, the invention relates to a cupin-nitroaldolase as described above, having the SEQ ID NO:2 (GtHNLmut) or (SEQ ID NO:4 (AcHNLmut).

According to a further embodiment, the cupin-nitroaldolase variant is A40H or A40R according to the numbering of SEQ ID NO:1, SEQ ID NO:3, or SEQ ID NO:5 to 10.

According to a further embodiment, the cupin-nitroaldolase variant is V42T or Q110H according to the numbering of SEQ ID NO: 1 or SEQ ID NO:3, or SEQ ID NO:5 to 10.

According to a further embodiment, the cupin-nitroaldolase variant is A40H V42T according to the numbering of SEQ ID NO:1 or SEQ ID NO:3, or SEQ ID NO:5 to 10.

According to a specific embodiment, the cupin-nitroaldolase variant is A40H V42T Q110H according to the numbering of SEQ ID NO: 1 or SEQ ID NO:3, or SEQ ID NO:5 to 10.

In a further aspect, the invention relates to a cupin-nitroaldolase as described above, characterized in that it comprises the amino acid sequence of the general formula: (X1)(X2)(X3)(X4)F(X5)PGAR(X6)(X7)WH(X8)HP(X9)G, wherein X1 is an A, V, L, F, Y, M, S, T, G, H, N, K or R residue, preferably it is an A, or N; X2 is any amino acid, preferably it is S, H, A, or T; X3 is a V, A, I, C, M, H, or T residue preferably it is a V; X4 is any amino acid, preferably it is T or R; X5 is any amino acid, preferably it is E; X6 is a T, S or N residue, preferably it is T; X7: is any amino acid, preferably it is A; X8: is a T, S, or I residues, preferably it is T; X9: is any amino acid preferably it is L; and wherein at least one of positions X1, or X3 is substituted by a H, K, R or T residue.

In a further aspect, the invention relates to a cupin-nitroaldolase as described above, characterized in that it comprises the amino acid sequence of the general formula: (X1)(X2)(X3)(X4)F(X5)PGAR(X6)(X7)WH(X8)HP(X9)G, wherein X1 is an A, V, L, F, Y, M, S, H, G, N, K or R residue, preferably it is an A, or N; X2 is any amino acid, preferably it is S, H, A, or T; X3 is a V, A, I, C, M, or T residue preferably it is V; X4 is any amino acid, preferably it is T or R; X5 is any amino acid, preferably it is E; X6 is a T, S or N residue, preferably it is T; X7: is any amino acid, preferably it is A; X8: is a T, S, or I residues, preferably it is T; X9: is any amino acid preferably it is L; and wherein at least one of positions X1, or X3 is substituted by a H, K, R or T residue.

In a further aspect, the invention relates to a cupin-nitroaldolase as described above, characterized in that it comprises the amino acid sequence of the general formula: (X1)(X2)(X3)(X4)F(X5)PGAR(X6)(X7)WH(X8)HP(X9)G, wherein X1 is an A, V, L, F, Y, M, S, T, G, N, K or R residue, preferably it is an A, or N; X2 is any amino acid, preferably it is S, H, A, or T; X3 is a V, A, I, C, M, or H residue preferably it is V; X4 is any amino acid, preferably it is T or R; X5 is any amino acid, preferably it is E; X6 is a T, S or N residue, preferably it is T; X7: is any amino acid, preferably it is A; X8: is a T, S, or I residues, preferably it is T; X9: is any amino acid preferably it is L; and wherein at least one of positions X1, or X3 is substituted by a H, K, R or T residue.

In a further aspect, the invention relates to a cupin-nitroaldolase as described above, characterized in that it comprises the amino acid sequence of the general formula: (X1)(X2)(X3)(X4)F(X5)PGAR(X6)(X7)WH(X8)HP(X9)G, wherein X1 is an A, V, L, F, Y, M, S, G, N, K or R residue, preferably it is A, or N; X2 is any amino acid, preferably it is S, H, A, or T; X3 is a V, A, I, C, or M residue preferably it is V; X4 is any amino acid, preferably it is T or R; X5 is any amino acid, preferably it is E; X6 is a T, S or N residue, preferably it is T; X7: is any amino acid, preferably it is A; X8: is a T, S, or I residues, preferably it is T; X9: is any amino acid preferably it is L; and wherein at least one of positions X1, or X3 is substituted by a H, K, R or T residue.

In a further aspect, the invention relates to a cupin-nitroaldolase as described above, characterized in that it comprises the amino acid sequence of the general formula: (X1)(X2)(X3)(X4)FEPGARTAWHTHPLG, wherein X1 is an A, V, L, F, Y, M, S, T, G, H, K, N or R residue, preferably it is an A, or N; X2 is any amino acid, preferably it is S, H, A, or T; X3 is a V, A, I, C, M, H, or T residue preferably it is V; X4 is any amino acid, preferably it is T or R; and wherein at least one of positions X1, or X3 is substituted by a H, K, R or T residue.

In a further aspect, the invention relates to a cupin-nitroaldolase as described above, characterized in that it comprises the amino acid sequence of the general formula: (X1)(X2)(X3)(X4)FEPGARTAWHTHPLG, wherein X1 is an A, V, L, F, Y, M, S, T, G, H, or N residue, preferably it is an A, or N; X2 is any amino acid preferably it is S, H, A, or T; X3 is a V, A, I, C, M, H, or T residue preferably it is V; X4 is any amino acid preferably it is T or R; and wherein at least one of positions X1, or X3 is substituted by a H, K, R or T residue.

In a further aspect, the invention relates to a cupin-nitroaldolase as described above, characterized in that it comprises the amino acid sequence of the general formula: (X1)(X2)(X3)(X4)FEPGARTAWHTHPLG, wherein X1 is an A, V, L, F, Y, M, S, T, G, H, K, N or R residue, preferably it is an A, or N; X2 is any amino acid, preferably it is S, H, A, or T; X3 is V; X4 is any amino acid, preferably it is T or R; and wherein at least one of positions X1, or X3 is substituted by a H, K, R or T residue.

In a further aspect, the invention relates to a cupin-nitroaldolase as described above, characterized in that it comprises the amino acid sequence of the general formula: (X1)(X2)(X3)(X4)FEPGARTAWHTHPLG, wherein X1 is an A, V, L, F, Y, M, S, G, N, K or R residue, preferably it is A, or N; X2 is any amino acid, preferably it is S, H, A, or T; X3 is a V, A, I, C, or M residue preferably it is V; X4 is any amino acid, preferably it is T or R; and wherein at least one of positions X1, or X3 is substituted by a H, K, R or T residue.

In a further aspect, the invention relates to a cupin-nitroaldolase as described above, characterized in that it comprises the amino acid sequence of the general formula: (X1)(X2)(X3)(X4)FEPGARTAWHTHPLG, wherein X1 is an A, or N residue, preferably it is A; X2 is any amino acid, preferably it is S, H, A, or T; X3 is a V, A, I, C, or M residue preferably it is V; X4 is any amino acid, preferably it is T or R; and wherein at least one of positions X1, or X3 is substituted by a H, K, R or T residue.

In a further aspect, the invention relates to a cupin-nitroaldolase as described above, characterized in that it comprises the amino acid sequence of the general formula: (X1)(X2)(X3)(X4)FEPGARTAWHTHPLG, wherein X1 is an A, or N residue, preferably it is A; X2 is any amino acid preferably it is S, H, A, or T; X3 is V; X4 is any amino acid preferably it is T or R; and wherein at least one of positions X1, or X3 is substituted by a H, K, R or T residue.

In a further aspect, the invention relates to a cupin-nitroaldolase having one or more of following mutations A40H, A40R, V42T and/or Q110H according to the numbering of SEQ ID NO: 1 or 3 or SEQ ID NO:5 to 10.

In a further aspect, the invention relates to an isolated polynucleic acid molecule encoding a cupin-nitroaldolase as described above.

In a further aspect of the present invention an isolated polynucleic acid molecule is provided encoding a cupin-nitroaldolase as described above. The polynucleic acid may be DNA or RNA. Thereby the modifications which lead to encoding the inventive cupin-nitroaldolase as described above are carried out on DNA or RNA level. This isolated polynucleic acid molecule is suitable for the production of inventive cupin-nitroaldolase as described above on a large scale.

In a further aspect, the invention relates to a vector comprising an isolated DNA molecule as described above.

The vector comprises all regulatory elements necessary for efficient transfection as well as efficient expression of proteins. Such vectors are well known in the art and any suitable vector can be selected for this purpose.

A further aspect of the present invention relates to a recombinant non-human cell which is transfected with an inventive vector as described above. Transfection of cells and cultivation of recombinant cells can be performed as well known in the art. Such a recombinant cell as well as any therefrom descendant cell comprises the vector. Thereby a cell line is provided which expresses the inventive cupin-nitroaldolase protein either continuously or upon activation depending on the vector.

In a further aspect, the invention relates to a culture obtained by culturing the recombinant cell as described above.

The recombinant cells may be cultured in the presence of a metal ion, preferably in the presence of iron or manganese.

In a further aspect, the invention relates to a cupin-nitroaldolase recovered from the culture as described above. Specifically, the protein can be isolated by disrupting the cells and recovering the protein from the supernatant.

In a further aspect, the invention relates to a method for producing a cupin-nitroaldolase, comprising recovering the cupin-nitroaldolase from the culture as described above. Said isolation or purification from the cell culture can be performed by methods known in the art. Specifically, affinity chromatography, anion-exchange chromatography and size exclusion chromatography can be used to isolate said protein.

In a further aspect, the invention relates to a cupin-nitroaldolase produced as described above, which is incubated with a solution of metal salts in buffer, for example FeCl₂, MnCl₂, CoCl₂, NiCl₂, CuCl₂ or ZnCl₂ by methods know to the art.

In a further aspect, the invention relates to a cupin-nitroaldolase produced as described above, which in which the metal is exchanged in vitro to manganese, iron, nickel, cobalt, copper or zinc by methods known to the art.

EXAMPLES

The Examples which follow are set forth to aid in the understanding of the invention but are not intended to, and should not be construed to limit the scope of the invention in any way. The Examples do not include detailed descriptions of conventional methods, e.g., cloning, transfection, and basic aspects of methods for overexpressing proteins in microbial host cells. Such methods are well known to those of ordinary skill in the art.

Example 1. Protein Production

The gene (gene ID: NC_012483) encoding a hypothetical protein from Acidobacterium capsulatum ATCC 51196 (Uniprot C1F951, Ward et al., Appl. Environ. Microbiol (2009) 75, 2046) was ordered codon-optimized for E. coli (GeneArt, Life Technologies, Carlsbad, Calif., USA) (named AcHNL in the following). The coding region was flanked by the NdeI and HindIII restriction sites, which were used to clone the gene into the expression vector pET26b(+) (Novagen, Merck KGaA, Darmstadt, Germany).

The sequence encoding AciX9_0562 (gene ID: 322434201) corresponding to GtHNL from Granulicella tundricula, which is codon-optimized for E. coli, was previously cloned into the expression vector pET26b(+) (Hajnal et al., FEBS J. 2013 280(22):5815-28). GtHNL was used as template for semi-rational protein design, in more detail, site-saturation mutagenesis. The mutations A40H, V42T and Q110H resulted in improved variants. The best amino acid exchanges at position A40, V42 and Q110 were combined by site-directed mutagenesis. The same amino acid exchanges at position A40, V42 and Q110 were subsequently also introduced in the sequence of AcHNL by site-directed mutagenesis.

E. coli BL21-Gold(DE3) was used as expression host (Stratagene, La Jolla, Calif., USA). The cells were grown in LB (lysogeny broth, Lennox) medium (Carl Roth GmbH, Karlsruhe, Germany) supplemented with kanamycin sulphate (40 mg/L final concentration). Expression of recombinant protein was initiated by addition of 0.5 mM IPTG (isopropyl 3-D-1-thiogalactopyranoside) to OD600˜0.8 cultures, and cultivation was continued at 25° C. for 20 h. All enzymes were grown with manganese present in the expression medium. Routinely, 100 μM of MnCl₂ was added concomitantly with the induction. The cells were harvested, resuspended in cold buffer (50 mM potassium phosphate buffer pH 6.0) and disrupted by sonification (Branson Sonifier S-250, set to 80% duty cycle, and output control 7) two times for 3 min, cooled on ice. The cell lysate was centrifuged for an hour at 50,000 g to remove unbroken cells and insoluble material. The cell free lysate was filtered through a 0.45 m syringe filter and if necessary concentrated to the desired concentration using Vivaspin 20 Centrifugal Filter Units (10,000 Da molecular-weight cut-off; Sartorius). The lysate was aliquoted and frozen until further use. Protein expression was monitored by standard SDS-PAGE. AcHNL and GtHNL as well as the variants of GtHNL and AcHNL containing the amino acid substitutions A40H, A40R, V42T and Q110H were expressed in high yield as soluble protein in E. coli BL21-Gold(DE3) reaching yields of >50% of total soluble protein.

The purification procedure was adapted from a protocol published for GtHNL applying anion-exchange chromatography and size exclusion chromatography (Hajnal et al., 2013). In short, the cells were lysed by sonication in Buffer A (50 mM Bis-Tris/HCl, pH 6.8 or 6.9, 50 mM NaCl), and the cleared lysates were loaded on a QSepharose anion-exchange column (HiTrap™ Q FF 5 mL, GE Healthcare, Uppsala, Sweden). The proteins were eluted with 10% buffer B (50 mM Bis-Tris/HCl, pH 6.6, 1 M NaCl). Size exclusion chromatography was performed on a Superdex 75 HiLoad 16/600 column (GE Healthcare, Uppsala, Sweden) pre-equilibrated with 50 mM NaPi, pH 7.0, 100 mM NaCl. Fractions containing the proteins of interest were pooled, and the buffer was exchanged to 50 mM KPi, pH 6.0 on PD-10 columns (GE Healthcare, Uppsala, Sweden). Protein concentrations of cell free lysates were routinely determined using the Bradford assay (Biorad, Hercules, Calif., USA). Concentrations of purified proteins were determined with a Nanodrop spectrophotometer (model 2000c, Peqlab, Erlangen, Germany) at 280 nm applying an extinction coefficient calculated based on the amino acid sequence using Protparam. The protein was stored at −80° C. or −20° C. until further use. The successful incorporation was confirmed by inductively coupled plasma/optical emission spectroscopy (ICP-OES).

Example 2. Nitroaldol Reaction in a Biphasic System

The ability of these enzymes to catalyze the synthesis of 3-nitroalcohols was examined using a two-phase system consisting of benzaldehyde and nitromethane dissolved in MTBE as organic phase and an aqueous phase comprising cell free lysate or purified enzymes in phosphate buffer at pH 6.

Organic solvent (500 μL MTBE) containing internal standard (0.2% 1,3,5-triisopropylbenzene, IS), benzaldehyde (20 μmol, BA) and nitromethane (1 mmol, NM) was mixed with 500 μL of 50 mM potassium phosphate buffer, pH 6, containing either cell free lysate (˜0.5-3 mg of total protein containing ˜50% cupin nitroaldolase) or purified enzyme (˜0.3-1.5 mg). As negative controls either cell free lysate without enzyme, AcHNL expressed in the absence of MnCl₂, or just buffer with and without MnCl₂ were used. As positive control AtHNL (2-3 mg), which was ordered as synthetic gene and cloned into pET28a(+)-vector, expressed and purified as described by Asano's group (Fuhshuku et al., 2011), was used. Note that due to stability issues of AtHNL at lower pH, it was applied at pH 6.5. The mixture was incubated at 30° C. and 1200 rpm for 2-24 hours in an Eppendorf Thermomixer device. The reaction was stopped by centrifugation at 13,000 rpm for 5 minutes. Fifty μL of the organic phase were diluted with 450 μL of the HPLC solvent mixture and analyzed by chiral HPLC.

The results of the first cupin-nitroaldolase catalyzed Henry reaction with benzaldehyde and nitromethane are shown in FIG. 2 and Table 1.

TABLE 1 Conversion and ee values related to FIG. 2. mg enzyme/mmol BA conversion [%] ee [%] 75 mg GtHNL-A40H/V42T/Q110H-lysate 69.9 97.5 75 mg GtHNL-A40H/V42T/Q110H-purified 68.5 97.3 75 mg AcHNL-WT-lysate 19.5 84.1 75 mg AcHNL-WT-purified 19.2 88.4 75 mg AcHNL-A40H/V42T/Q110H-purified 45.5 94.2 buffer 9.8 2.3 n.d.: not determined due to too low conversion

AcHNL showed a conversion of 19% and an ee of 88% after 6 h of incubation. GtHNL-A40H/V42T/Q110H, achieved almost 70% conversion and 97% ee. No difference was observed if cleared lysates or purified enzymes were applied. Thus, the purified enzymes are stable under the applied reaction conditions, but on the other hand do not need to be purified to obtain good to excellent ee values. All negative controls, AcHNL, which was expressed in the absence of MnCl₂, as well as MnCl₂ applied in buffer or in E. coli lysate without enzyme, were inactive. Thus, both the cupin protein and manganese are necessary in combination for nitroaldol activity.

To investigate further if also lower amounts of enzyme can achieve good conversions, if the ee can be further improved applying shorter reaction times, and if the ee is stable during longer incubation times the reactions with GtHNL-A40H/V42T/Q110H were repeated (FIG. 3 and Table 2).

TABLE 2 Conversion and ee values related to FIG. 3. Reaction time 2 h 24 h mg enzyme/mmol BA conv. [%] ee [%] conv. [%] ee [%] 62.5 mg GtHNL-A40H/V42T/ 52.0 98.4 72.6 94.1 Q110H 37.5 mg GtHNL-A40H/V42T/ 31.4 97.8 73.7 95.0 Q110H 15 mg GtHNL-A40H/V42T/ 17.0 95.5 67.7 93.4 Q110H 100 mg AtHNL 7.1 78.9 25.8 34.8 buffer 0.6 n.d. 7.8 0.2 n.d.: not determined due to too low conversion

Compared to values reported in literature for AtHNL (Fuhshuku et al., 2011) and also determined by us, the above mentioned newly discovered GtHNL triple variant is more active than AtHNL, and it shows a higher enantioselectivity.

Importantly, in contrast to AtHNL higher yields can be achieved with GtHNL-A40H/V42T/Q110H by longer incubation times losing almost no enantioselectivity.

Single variants of AcHNL and GtHNL were tested for their ability to catalyze the nitroaldol reaction.

TABLE 3 Conversion and ee values. mg enzyme/ mmol Conversion enzyme BA time [h] % ee (R) % AcHNL lysate 12.5 4  2.5 ± 0.3 57.9 ± 0.1 24 17.8 ± 0.2 55.5 ± 0.3 50 24 38.5 ± 0.7 77.3 ± 0.3 AcHNL purified 25 2  4.2 ± 0.2 78.8 ± 1.2 4  7.7 ± 0.1 80.3 ± 0.1 24 37.5 ± 1.8 78.9 ± 0.0 AcHNL-A40H lysate 12.5 4 33.3 ± 0.4 96.7 ± 0.1 24 73.4 ± 0.0 95.3 ± 0.1 AcHNL-A40H purified 25 2 62.9 ± 0.6 99.3 ± 0.0 4 72.7 ± 0.9 99.3 ± 0.0 24 74.0 ± 0.1 97.0 ± 0.1 AcHNL-A40R lysate 12.5 4 50.2 ± 0.5 97.4 ± 0.0 AcHNL-A40R purified 25 2 57.3 ± 0.9 98.5 ± 0.1 4 67.7 ± 0.3 98.3 ± 0.0 24 73.3 ± 0.2 96.0 ± 0.1 AcHNL-V42T lysate 50 24 48.9 ± 0.0 84.7 ± 0.1 AcHNL-Q110H lysate 50 24 17.2 ± 0.1 47.3 ± 0.1 AcHNL- 12.5 4 12.0 ± 0.4 89.5 ± 0.4 A40H/V42T/Q110H 24 50.4 ± 0.7 88.5 ± 0.3 lysate AcHNL- 25 2 11.4 ± 0.0 94.6 ± 0.0 A40H/V42T/Q110H 4 21.2 ± 0.3 93.9 ± 0.7 purified 24 66.3 ± 0.1 93.2 ± 0.1 GtHNL-A40H lysate 12.5 4 14.1 ± 0.4 90.8 ± 1.1 24 53.8 ± 0.6 89.7 ± 0.0 GtHNL-A40H purified 25 2 22.7 ± 0.1 96.3 ± 0.1 4 38.3 ± 0.5 96.7 ± 0.1 24 74.2 ± 0.0 95.5 ± 0.0 GtHNL-A40R lysate 12.5 4 50.1 ± 0.1 97.2 ± 0.0 24 74.0 ± 0.0 94.3 ± 0.1 GtHNL-A40R purified 25 2 55.9 ± 0.1 98.1 ± 0.0 4 70.5 ± 0.1 97.9 ± 0.1 24 75.2 ± 0.4 94.0 ± 0.2 GtHNL-V42T lysate 50 24 31.2 ± 0.1 74.1 ± 0.6 GtHNL-Q110H lysate 50 24 27.9 ± 1.1 68.1 ± 0.1 GtHNL-A40H/V42T 12.5 4  8.3 ± 0.1 85.0 ± 0.5 lysate 24 39.9 ± 0.7 83.7 ± 0.4 GtHNL- 12.5 4 11.8 ± 0.1 88.4 ± 0.2 A40H/V42T/Q110H 24 50.1 ± 0.2 88.0 ± 0.2 lysate GtHNL- 25 2 17.9 ± 0.8 95.3 ± 0.2 A40H/V42T/Q110H 4 30.6 ± 0.1 95.8 ± 0.1 purified 24 72.8 ± 0.4 94.5 ± 0.0

Example 3. Nitroethane Addition to Benzaldehyde

Purified AcHNL, AcHNL-A40H, AcHNL-A40R, AcHNL-A40H/V42T/Q11 OH, GtHNL-A40R and GtHNL-A40H/V42T/Q110H were tested for their ability to use nitroethane instead of nitromethane in the nitroaldol reaction. The reactions were performed as described in Example 2, with the difference that 1 mmol nitroethane instead of nitromethane was used.

The results of the first cupin-nitroaldolase catalyzed Henry reaction using purified enzyme with benzaldehyde and nitroethane are shown in FIG. 4 and Table 4.

TABLE 4 Conversion and ee values related to FIG. 4. mg enzyme/ mmol conversion ee (R, anti) purified enzyme BA time [h] [%] [%] AcHNL 500 2 20.3 ± 0.5 84.8 ± 0.3 4 33.9 ± 0.3 82.5 ± 0.1 24 77.4 ± 0.2 73.4 ± 0.2 AcHNL-A40H 125 2 16.4 ± 0.0 85.7 ± 0.2 4 30.0 ± 0.1 90.1 ± 0.3 24 66.0 ± 0.0 88.3 ± 0.1 AcHNL-A40R 125 2 22.3 ± 1.9 65.2 ± 4.5 4 36.7 ± 0.6 74.5 ± 0.2 24 73.1 ± 1.0 69.0 ± 0.8 AcHNL- 500 2 21.3 ± 2.1 89.1 ± 1.0 A40H/V42T/Q110H 24 77.2 ± 0.1 87.7 ± 0.1 GtHNL-A40R 125 2 26.9 ± 0.0 74.1 ± 0.6 4 43.8 ± 0.2 78.0 ± 0.2 24 74.8 ± 0.4 64.7 ± 0.5 GtHNL- 500 2 21.5 ± 1.0 76.7 ± 0.4 A40H/V42T/Q110H 4 36.4 ± 0.7 74.9 ± 0.1 24 78.4 ± 0.2 69.1 ± 0.0

The addition of nitroethane to benzaldehyde produces two new stereocenters simultaneously and a diastereomeric mixture of 2-nitro-1-phenylpropanol was obtained. After 2 h of reaction time, the anti/syn ratio of AcHNL was 2:1 and the enantiomeric excess of the anti isomer was 85%. Thus, the product mixture contains about 60% of the expected main product (1R,2S)-2-nitro-1-phenylpropanol. With the single variant, AcHNL-A40H, the proportion of (1R,2S)-2-nitro-1-phenylpropanol was further increased to 70%.

Example 4. Nitromethane Addition to Various Aldehydes

The reactions were performed as described in Example 2, with the modification that instead of benzaldehyde, either 2-Cl-benzaldehyde, hexanal or cyclohexanecarboxaldehyde were used.

TABLE 5 Conversion and ee values.

Conversion R Purified protein [%] ee (R) [%] 2-ClC₆H₄ GtHNL-A40R 97.3 ± 0.0 23.2 ± 0.1 GtHNL-A40H/V42T/Q110H 95.0 ± 0.4 56.3 ± 0.2 AcHNL-A40H 89.0 ± 0.9 83.1 ± 0.5 AcHNL-A40R 95.3 ± 0.2 80.0 ± 0.2 AcHNL-A40H/V42T/Q110H 90.6 ± 1.3 54.8 ± 0.2 cyclohexyl GtHNL-A40R 31.1 ± 0.8 47.0 ± 1.5 GtHNL-A40H/V42T/Q110H 86.3 ± 0.6 96.9 ± 0.1 AcHNL-A40H 48.2 ± 0.1 87.6 ± 0.5 AcHNL-A40R 39.7 ± 1.8 77.8 ± 1.4 AcHNL-A40H/V42T/Q110H 80.7 ± 1.3 95.7 ± 0.3 CH₃(CH₂)₄ GtHNL-A40R 92.0 ± 0.3 98.0 ± 0.0 GtHNL-A40H/V42T/Q110H 95.0 ± 0.0 99.3 ± 0.0 AcHNL-A40H 88.8 ± 0.2 98.9 ± 0.0 AcHNL-A40R 87.9 ± 0.2 98.2 ± 0.1 AcHNL-A40H/V42T/Q110H 93.7 ± 0.2 99.2 ± 0.0

Reaction conditions: aldehyde (20 mM) and nitromethane (1 M) in a biphasic system consisting of TBME and 50 mM KPi, pH 6.0, 1:1, reaction volume 1 mL, 30° C., 1200 rpm, 24 h. Purified enzyme (2.5 mg) was used.

Example 5. Metal-Dependence of Nitroaldol Reaction

For the removal of metal ions from GtHNL-A40H/V42T/Q110H, the purified protein (as described in Example 1) was dialysed against 20 mM 2,4-pyridinedicarboxylic acid monohydrate (PDCA) in 100 mM sodium acetate and 150 mM NaCl, pH 5.5, for 50 h, and subsequently dialysed against 20 mM Tris/HCl, pH 7.5, for 20 h. The resulting apoprotein was incubated with a 10-fold molar excess of either FeCl₂, MnCl₂, CoCl₂, NiCl₂ or ZnCl₂ for 2.5 h at room temperature. Unbound metal was removed and the buffer was exchanged to 50 mM KPi, pH 6.0, using PD-10 columns. Metal analysis was performed by ICP-OES.

The reactions were performed as described in Example 2.

TABLE 6 Conversion and ee values. mg enzyme/ Conversion mmol BA time [h] % ee % GtHNL- 25 4 28.5 ± 1.9 95.2 ± 0.5 A40H/V42T/Q110H(Mn)^(a) 100 24 75.8 ± 0.0 95.1 ± 0.2 GtHNL- 25 4  4.2 ± 0.4 60.7 ± 0.8 A40H/V42T/Q110H_Apo^(b) 100 24 36.1 ± 0.4 77.0 ± 0.3 GtHNL- 25 4 33.7 ± 0.6 96.2 ± 0.3 A40H/V42T/Q110H_Mn^(c) 100 24 75.7 ± 0.1 94.7 ± 0.2 GtHNL- 25 4 41.3 ± 1.1 97.2 ± 0.2 A40H/V42T/Q110H_Fe^(c) 100 24 75.7 ± 0.1 95.7± 0.1 GtHNL- 25 4 19.0 ± 0.7 91.4 ± 0.0 A40H/V42T/Q110H_Zn^(c) 100 24 75.7 ± 0.2 95.2 ± 0.0 GtHNL- 25 4 44.8 ± 0.5 97.3 ± 0.0 A40H/V42T/Q110H_Co^(c) 100 24 75.7 ± 0.0 95.7 ± 0.2 GtHNL- 25 4 23.8 ± 1.9 93.4 ± 0.4 A40H/V42T/Q110H_Ni^(c) 100 24 75.6 ± 0.1 95.1 ± 0.0 ^(a)GtHNL-A40H/V42T/Q110H was expressed in the presence of MnCl₂ in the expression medium. ^(b)Bound metal was removed by the chelator 2,4-pyridinedicarboxylic acid. Traces of manganese were detected by ICP-OES. ^(c)Apoprotein was incubated with the respective metal salts.

Example 6. Other Cupin-HNLs with Nitroaldol Activity

Several other cupins with sequence identities to SEQ ID NO: 1 and 3 between 58 and 84% (Wiedner, R.; et al., Comp. Struct. Biotechnol. J., (2014), 10, 58) were tested for their ability to catalyze the nitroaldol reaction. Protein expression and preparation of cell-free lysates were performed as described in Example 1.

TABLE 7 Cupin-HNLs with high to moderate sequence identity (seq id) to SEQ ID NO: 1 and 3. % seq % seq SEQ Uniprot id to id to Sequen- ID Accession SEQ ID SEQ ID cing NO number Organism NO: 1 NO: 3 date 5 B8ENI4 Methylocella silvestris 84 81 2008¹ BL2 6 A5G162 Acidiphilium cryptum 74 76 2007  JF-5 7 C6D499 Paenibacillus sp. JDR-2 78 79 2009  8 C1D3E9 Deinococcus deserti 77 77 2009² (strain VCD115/DSM 17065/LMG 22923) 9 A6U7V5 Sinorhizobium medicae 73 73 2007³ WSM419 10 F8IF03 Alicyclobacillus 58 60 2011⁴ acidocaldarius subsp. acidocaldarius Tc-4-1 ¹Chen Y, Crombie A, Rahman MT, Dedysh SN, Liesack W et al. (2010) J Bacteriol 192: 3840-3841. ²de Groot A, Dulermo R, Ortet P, Blanchard L, Guerin P et al. (2009) JPLoS Genet 5. Available: http://dx.plos.org/10.1371/journal.pgen.1000434. ³Reeve W, Chain P, O'Hara G, Ardley J, Nandesena K et al. (2010) Genomic Sci 2: 77-86. ⁴Chen Y, He Y, Zhang B, Yang J, Li W et al. (2011) J Bacteriol 193: 5602-5603.

The reactions were performed as described in Example 2.

TABLE 8 Conversion and ee values mg enzyme/ Enzyme mmol in lysate BA Time [h] Conversion [%] ee [%] AcHNL 500 24 86.7 ± 1.0 81.0 ± 0.5 B8ENI4 73.3 ± 1.0 69.1 ± 0.1 A5G162 75.3 ± 2.8 55.0 ± 0.5 C6D499 82.0 ± 0.2 71.5 ± 0.4 C1D3E9 82.9 ± 0.2 85.2 ± 0.1 A6U7V5 69.2 ± 4.1 60.3 ± 0.2 F8IF03 72.4 ± 1.5 75.5 ± 0.4

Example 7. Nitroaldol Reaction in an Aqueous System

The ability of cupin-nitroaldolases to catalyze the synthesis of β-nitroalcohols was examined using a one-phase aqueous system consisting of 5 μmol benzaldehyde, 0.3 mmol nitromethane, and 0.375 mg of total lysate protein (containing ˜50% cupin-nitroaldolase) or 0.5 mg of purified protein in 500 μL of 50 mM potassium phosphate buffer at pH 6. After incubation at 30° C. and 1200 rpm for 1 h, the reaction was stopped by extraction with 500 μL MTBE containing 0.2% internal standard (1,3,5-triisopropylbenzene, IS). Fifty μL of the organic phase were diluted with 450 μL of the HPLC solvent mixture and analyzed by chiral HPLC.

Results of the first cupin-nitroaldolase catalyzed Henry reaction in an aqueous system with benzaldehyde and nitromethane are shown in Table 9.

TABLE 9 Conversion and ee values. mg enzyme/ time Conversion mmol BA [h] % ee % GtHNL-A40H/V42T/Q110H 37.5 1 71.1 94.9 lysate GtHNL-A40H/V42T/Q110H 200 1 74.8 82.2 purified 

The invention claimed is:
 1. A cupin-nitroaldolase variant protein characterized in that it comprises the amino acid sequence of the general formula: (X1)(X2)(X3)(X4)F(X5)PGAR(X6)(X7)WH(X8)HP(X9)G, wherein X1 is an A, V, L, F, Y, M, S, T, G, H, N, K, or R residue; X2 is any amino acid; X3 is a V, A, I, C, M, H, or T residue; X4 is any amino acid; X5 is any amino acid; X6 is a T, S or N residue; X7: is any amino acid; X8: is a T, S, or I residues; X9: is any amino acid; and wherein at least one of positions X1, or X3 is substituted by a H, K, R or T residue.
 2. The cupin-nitroaldolase variant protein according to claim 1, wherein in the general characterized in that it comprises the amino acid sequence of the general formula (X1)(X2)(X3)(X4)F(X5)PGAR(X6)(X7)WH(X8)HP(X9)G, X1 is an A or N residue; X2 is an S, H, A or T residue; X3 is a V residue; X4 is a T or R residue; X5 is an E residue; X6 is a T residue; X7: is an A residue; X8: is a T residue; X9: is an L residue; and wherein at least one of positions X1, or X3 is substituted by a H, K, R or T residue.
 3. The cupin-nitroaldolase variant protein according to claim 1, wherein the cupin-nitroaldolase is at least 85% identical to the respective wild type enzyme.
 4. The cupin-nitroaldolase variant protein according to claim 2, wherein the cupin-nitroaldolase is at least 85% identical to the respective wild type enzyme.
 5. The cupin-nitroaldolase variant protein according to claim 1 having one or more of the following mutations: A4OH, A4OR, V42T and/or Q110H according to the amino acid numbering of SEQ ID NO: 1 or 3 or SEQ ID NO: 5 to
 10. 6. The cupin-nitroaldolase variant protein according to claim 1 having the amino acid sequence of SEQ ID NO: 2 or
 4. 